The story starts back in 2001 when the cities of the Tri-Valley received funding to upgrade their traffic communication systems. This three million dollar grant allowed Pleasanton to upgrade signal controllers, office equipment, and parts of the main communication network that enabled traffic signal coordination to be possible. Then, in 2002, the first signal coordination began.
For coordination to occur, an engineer creates a pattern that traffic signals along a roadway will follow. These patterns have to take into account such variables as traffic volume, peak hours, intersection size, cycle time, and even the time it takes for pedestrians to cross. The problem with the original coordination patterns put in place in 2002 was a lack of follow-up to see if the patterns were working well.
Within the last two years, Pleasanton's Traffic Engineering department has been giving more attention to optimizing the signal coordination patterns and making sure they're working effectively. Their goal is to get as many vehicles cleared through a green light as possible. This is more daunting than it seems. Pleasanton's streets are not laid out as a grid so signal coordination can get complex. Preference is given to the direction with the most amount of cars and this can change depending upon location. For example, cars pour into the Hacienda Business Park from both directions on Santa Rita Road in the mornings. A driver not following the same path as the rest of this large volume of traffic can end up getting a red light after passing the streets that enter the business park.
Cycle times also play an important role. A cycle is the amount of time it takes for all the lights at an intersection to change at least once. All the signals along a coordinated street need to use the same cycle time. It is this that can cause a driver to feel like they've been waiting at a light for too long. The traffic on a minor street may be light, but the signals still have to be synchronized with the longer cycle time of a major intersection.
To illustrate this, Hopyard Road runs coordinated signals. At the Stoneridge and Hopyard intersection, both are major streets that need about 60 seconds each to let cars through, or 120 seconds total. Further south at Hopyard and Valley Trails, Hopyard needs 60 seconds but Valley Trails only needs about 20 seconds. The intersection still has to use 120 total seconds in order to be coordinated with Stoneridge and Hopyard. That leaves about 40 seconds of extra time which is given to Hopyard. So a driver on Valley Trails could end up waiting for a good amount of time before getting a green light. To alleviate this side-effect of signal coordination, the Traffic department limits the time Valley Trails and Hopyard are coordinated to two hours in the morning and evening.
Several major roadways in Pleasanton see signal coordination active from 7AM until 7PM on weekdays. These are Santa Rita Road, Valley Avenue, First Street/Stanley Avenue, and Hopyard Road. Also, Santa Rita sees coordination from 10AM to 7PM on weekends.
Stoneridge Drive currently does not run coordinated signals. I was surprised to learn that the reason why is because the traffic volume through Hacienda Business Park is too low for coordination to be beneficial. There are not enough vehicles traveling the street's length from 680 to Santa Rita or Santa Rita to 680. Instead, the majority of traffic enters the business park and disperses throughout Hacienda's many side streets. Additionally, the large intersections on Stoneridge have to run long cycle times so that pedestrians can safely cross. Coordinating these would end up causing long and unnecessary wait periods on side streets.
In contrast to Stoneridge Drive, traffic volumes on First Street are too high and the intersections are too small. As a result, traffic signal coordination on First Street provides only limited benefit.
Synchronization is important, but is only a single part of the overall solution to fix Pleasanton's traffic congestion problems. Mr. Tassano gave me his take on what would help relieve traffic congestion. The solutions depend upon the locations. Some locations need larger intersections while in other locations, giving drivers more route options would help. Lastly, newer vehicle detection equipment would allow Pleasanton's traffic engineers to reduce wait times on side streets.
Traffic signal synchronization is an ongoing optimization process that, given enough time and attention, continues to improve. Look for future improvements in signal coordination during the school rush hour.